Wednesday, March 30, 2016


cURL is a very useful library written in C which allows you to do a lot of network jobs like calling HTTP REST API, etc.

Recently, I need to add a timeout value to my program which uses cURL, and I found there is a convenient option called CURLOPT_TIMEOUT_MS:
which could add a timeout in milliseconds to a cURL connection.

But I found that whenever I set the timeout less than 1000 milliseconds, the cURL connection timeouts immediately after it is performed. After googling online for a while, I found the issue is caused by:
If libcurl is built to use the standard system name resolver, that portion of the transfer will still use full-second resolution for timeouts with a minimum timeout allowed of one second. The problem is that on Linux/Unix, when libcurl uses the standard name resolver, a SIGALRM is raised during name resolution which libcurl thinks is the timeout alarm.
One quick fix for this problem is to disable signals using CURLOPT_NOSIGNAL. For example:
curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1);
// timeout in milliseconds
curl_easy_setopt(curl, CURLOPT_TIMEOUT_MS, timeoutMilliseconds);
// Perform the request, res will get the return code
res = curl_easy_perform(curl);
// Check for errors
       throw "timeout";
else if(res != CURLE_OK)
       throw "error info: "+std::string(curl_easy_strerror(res);


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